Diaphragm for electrostatic loud-speakers



Oct. 28, 1952 T. LINDENBERG ET AL DIAPHRAGM FOR ELECTROSTATICLOUD-SPEAKERS 14, 1948 2 SHEETSSHEET 1 Filed Dec.

R 2. 7 J 2 F P 4 M F m m R J. Wu. J F m m fl i M |l+|l u, L 5 M 4 m 3 nu 1 1 u r" m m\ I F H 1 5 :3 T 1} 6 L l Lr l .n a Z Z- T n 3 ma 1952 T.LINDENBERG ET AL 2,615,994

DIAPHRAGM FOR ELECTROSTATIC LOUD-SPEAKERS Filed Dec. 14, 1948 2SHEETS-SHEET 2 .l MED.

-!r 133:: Von/we INVERSE FEED BACK 400 V DC FIG. 7.

Theodor .Zizzdezeber Granf 7. Laayiz Zz'n Patented Oct. 28, 1952DIAPHRAGM FOR ELECTROSTATIC,

LOUD-SPEAKERS Theodore Lindenberg, Manhasset, and GrantT. Lou'ghlin,Crestwood; N. Y.

Application December 14, 1948, Serial No. 65,236

3 Claims.

The present invention relates to sound producers and reproducers, suchas those used in radio reception, and has reference to the type known aselectrostatic sound reproducers or loud speakers.

The ability to reproduce the higher tonal range of recorded or radio,music is most difiicult in the final electro-acoustic step of movingreasonable volumes of air in exact accord with the audio frequency inputto the reproducer. It is known that the smaller, lighter and stifferdiaphragms are most efheient and produce the lowest distortion. Theconventional loud speaker (a paper cone, driven from a coil at its apex)hasevolved as the natural commercial design incorporating the abovequalities.

This cone actually moves the body of air on both its sides, after themanner of a piston, for low frequencies, but at higher frequencies, theload of air upon it becomes too great and, under the driving force fromthe voice coil, the paper gives and the area of the cone moves insections. This is known as break up and causes a loss of efficiency asless air is modulated. More important, distortion is introduced becauselisteners hear this breaking up as added harmonics, or partials,introduced by the paper along with the pure tone that, of course, isreduced in volume. This fact is known and it is customary in theconstruction of such loud speakers to put a few annular embossed ringsin the cone paper as a par tial control in minimizing the distortion.

The highest sound frequencies normally radiate only from the very centerof the conewithin a half inch or so of the voice coil, and are not onlyweak and distorted but are highly directional, that is, can be heardwell only when the listener occupies the principal aXis of the speaker.

Loud speakers or sound reproducers of this type start to introducedistortion above 1000 cycles per second and usually fall off rapidly inefficiency above 3000 cycles per second. Together with these losses, thedistortion takes the form of a papery quality which modifiesobjectionably the reproduction;

The best commercially available loud speakers today, which are used 'forlistening standards in broadcast and recording studios, operateon adifferent principle. The audio signal is divided and a speciallydesigned speaker produces high'frequencies only. The diaphragm thereofis made very small and light to gain efficiency. and attempt to hold thepiston actionto a-higherafrequency before break-upoccurs.

"In one of 'such' speakers; the high frequency cone is very small and isplaced within the apex of the larger low-frequency speaker. In anothercommercially used speaker, the diaphragm is composed of thin sheetmetal'and is dome-shaped for stiffness. This dome-shaped diaphragm isquite small and is connected by an acoustic loading throat to aplurality of small exponential horns to achieve a non-directionaleffectby projecting the sound over considerable angular area. Thisarrangement has also been designed to cocupy the center of aconventional low-frequency cone speaker; although in larger size, thenest of exponential horns is usually placed above a low-frequencyb-aflle.

In these loud speakers, the attempts to reproduce accurately highfrequency sound depend on translating the electromotive force of thevoice coil to a body of air by means of stiff diaphragms. Suchdiaphragms must also be of negligible mass toprevent losses at highfrequencies attributable to inertia. This compromise still leaves a gapbetween natural true reproduction and the original sound. The trainedear can distinguish easily between the metallic diaphragm and the papercone when the speakers are concealed behind a thin curtain, and even theuntrained ear objects, as it were, to such so-called high fidelity,particularly when the speakers operate at high volume.

It is an object of the invention to provide an electrostatic orcondenser-type loud speaker utilizing a flexible sheet-type diaphragm.

Another object of the invention is to provide an electrostatic loudspeaker adapted for the true reproduction of the higher acoustic range,covering frequencies above 1000 cycles per second up to the upper limitof human audibility.

It is another object of the invention to, provide an electrostatic loudspeaker adapted to be employed as a supplement to existing loud speakerssuitable for relatively low frequency sound reproduction, whereby toprovide true :to'nal effects or characteristics throughout th entirerange of frequencies audibleto the human sense of hear- 1 s- A furtherobject is, to provide an electrostatic loud speaker having a diaphragmespecially adapted for the clear reproduction of sound found in theupper frequency spectrum and embodying thin sheets of a metallic foilseparated by a thin flexible dielectric, such as paper or rubber.

Another object is to provide a loud speaker diaphragm of this characterwhich is capable of being driven evenlyiand in phase over its entirearea.

Still a further object is to provide a structurally simple andinexpensive diaphragm for electrostatic loud speakers which willgenerate undistorted acoustic output in the upper frequencies of audiblesound and wherein the diaphragm comprises two thin fiexib1e sheets of ametallic foil which are secured together and separated electrically bypaper sheets arranged at their edges.

For a further understanding of the invention, including additionalobjects and advantages thereof, reference is to be had, to the followingdescription and the accompanying drawings, wherein:

Fig. 1 is a front elevational view of my improved sheet-type diaphragmfor use in electrostatic loud speakers;

Fig. 2 is an edge elevational view thereof. In this figure, thethicknesses of the several parts of the diaphragm have been exaggeratedfor clarity of illustration.

Fig. 3 is a transverse vertical sectional View taken on the line 33 ofFig. 1;

Fig. 4 is a fragmentary detail horizontal sectional view taken on theline 44 of Fig. 1;

Fig. 5 is a perspective view disclosing my improved diaphragm in asomewhat curved arouate state to widen the angles of sound propagationtherefrom;

Fig. 6 is a transverse horizontal sectional view disclosing the mannerin which m improved diaphragm, which is adapted for high frequency soundreproduction, may be incorporated in the cone diaphragm of a lowfrequency reproducer;

Fig. 7 is a Wiring diagram showing one method of connecting the speakerof the present invention with an output circuit;

Fig. 8 is a similar view of another method of connecting the speaker ofthe present invention with an output circuit in which is involved alow-frequency sound reproducer.

Referring to the drawings, and especially Figs. 1 and 2 thereof, myimproved diaphragm has been shown as being composed of thin capacitativesheet of metallic foil, as indicated at l and 2. In this regard,aluminum foil may be employed having a thickness of the order of .0002of an inch, although other conducting materials may be employed insheets of different thicknesses. The foil sheets I and 2 are flatly andadhesively mounted on paper backing sheets indicated at 3 and 4, and theadjoining surfaces of the sheets 3 and 4 are united and maintained inspaced relationship from each other by the employment of insulatingspacer strips 5 around the edges only of the sheets. The latter producean air chamber 6 between the capacitative sheets 3 and a. to separatethe latter primarily for damping purposes. These strips and sheets areadhesively joined together in fixed unitary relationship, providing athin flexible diaphragm. If desired, the sheets I and 2 may be providedwith laterally projecting tabs 1 for the attachment of conductor wires 8of a circuit from which sound may be reproduced from electrical energy.Due to rapid differences in potential levels of the charges on themetallic sheets l and 2, and as induced into the material or surfaces ofthe paper dielectric sheets 3 and 4, the diaphragm will vibrate toproduce sound. The vibrations are accurate facsimiles of the changes inpotential and the noise normally parasitic upon the sound of theordinary loud speaker when reproducing high frequencies is absent whenour improved diaphragm is utilized. The diaphragm may be suitablysupported by having its marginal edges disposed in a supporting frameindicated at 9.

The diaphragm is, of course, subject to certain variation. As shown inFig. 5, our improved diaphragm, which is indicated in its entirety bythe letter D, may be curved both longitudinally and transversely,whereby to widen its directional characteristics for room propagation ofsound- In Fig. 6, we have shown our improved diaphragm as being arrangedin combination with a low frequency loud speaker. In this arrangement,the diaphragm D functions to reproduce high frequency sound,particularly sound above 1000 cycles per second, and is employed inconjunction with a low frequency cone l4 of conventional constructionand operation. As shown, the combined loud speaker unit is provided withthe usual rigid perforated outer cone ring, which carries a flexibleannular suspension I6 for the apex region of the cone I4. The apex ofthe cone is formed with the conventional tubular extension ll which islocated in a circular slot [8 provided in an annular magnet 19. Circuitconductors are shown at 26 and these conductors lead to the voice coil2| on the cone extension M. It will be observed that the diaphragm D isdisposed in the center at the front of the cone M and is stationarilymounted in any suitable manner in this position. Conductors 8b aredisclosed as leading to the metallic plates or elements of the diaphragmD.

In the operation of the diaphragm D, it is desirable to permit only thefrequency spectrum above 1000 cycles per second to enter the diaphragm.This may be accomplished by the circuit disclosed in Fig. '7 in which byreducing the fixed condenser 6| to a small value, or by filtering thelow frequencies out in the amplifier used to drive the diaphragm, thelatter will respond to the desired high frequencies. The wiring diagramof Fig. 7 is applicable to the diagram of Fig. 1, whereas in Fig. 8, awiring diagram has been illustrated suitable for use in connection withthe apparatus of Fig. 6 in which my improved high frequency soundreproducer is combined with a conventional sound reproducer of arelatively low-frequency type.

While we have herein described and shown various illustrativeembodiments of the invention, it is to be understood that the latter isnot limited thereto, but may comprehend other parts, details andarrangements of features without departing from the spirit of theinvention.

We claim:

1. An electrostatic sound reproducer comprising a diaphragm embodyin apair of spaced flexible base sheets of dielectric composition, flexiblemetal foil plates directly applied to the outer surfaces only of saidbase sheets to form component parts thereof, and spacing strips offlexible material positioned between and uniting the marginal edgeportions only of said base sheets, said spacing strips serving tophysically separate said base sheets from each other and producing ashallow air chamber therebetween.

2. A composite diaphragm for electrostatic sound reproducers, comprisinga pair of juxtaposed relatively spaced thin ilexible base sheets ofdielectric composition, correspondingly thin and flexible metal foilplates directly applied and adhesively'secured to the outer surfacesonly of said base sheets to form component parts thereof, and spacingstrips positioned between and adhesively secured to the inner adjoiningmarginal edge portions of said base sheets to unite the latter inassembled relationship and to form between said base sheets a shallowair chamber, the latter possessing a thickness corresponding with thatof the spacing strips.

3. A composite sound-wave generating diaphragm for electrostatic soundreproducers, comprising a pair of parallel closely spaced relativelythin flexible base sheets of paper, correspondingly thin flexiblemetallic foil plates applied to the outer surfaces of said base sheetsto form component synchronously flexing parts thereof, and flexiblespacing means positioned between and uniting the inner adjoiningmarginal edge portions of said base sheets in a manner forming betweensaid sheets a shallow air chamber.

THEODORE LINDENBERG. GRANT T. LOUGHLIN.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 963,934 Ort etal July 12, 1910 1,433,224 Parkin Oct. 24, 1922 1,674,683 Hahnemann June26, 1928 1,850,855 Thomas Mar. 22, 1932 1,889,748 Gruschke Dec. 9, 19322,043,241 Ever June 4, 1936 2,126,436 Williams Aug. 9, 1938 FOREIGNPATENTS Number Country Date 349,630 Great Britain June 4, 1931

